Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulose materials, such as biofuels. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulose material, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulose materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. During the reduction of alkanoic acid, e.g., acetic acid, other compounds are formed with ethanol or are formed in side reactions. These impurities limit the production and recovery of ethanol from such reaction mixtures. For example, during hydrogenation, esters are produced that together with ethanol and/or water form azeotropes, which are difficult to separate. In addition when conversion is incomplete, unreacted acid remains in the crude ethanol product, which must be removed to recover ethanol.
Excess of hydrogen is used to increase the yield of ethanol production in converting carbonaceous feedstock into low-molecular weight alcohols. Due to the use of excess amounts of hydrogen, it is beneficial to recycle the unreacted hydrogen back to the reactor. However, additional gases, such as methane, ethane, nitrogen, carbon monoxide, and carbon dioxide, which would build up in the reactor when hydrogen is recycled, are also formed during the reaction.
EP2060555 describes hydrogenating esters to alcohols and separates a hydrogen gas recycle stream in an alcohol separation zone.
EP2069269 describes hydrogenating acetic acid to hydrocarbons and a flasher for separating the crude mixture into a vapor fraction comprising carbon monoxide, carbon dioxide, methane, propane, water, and unreacted hydrogen. The vapor fraction is recycled to the reactor by passing through a carbon dioxide separator.
However, a need remains for improving the processes for controlling non-condensable gas from the hydrogenation of acetic acid to increase production of ethanol.